In the prior art, salts are conventionally produced in crystallizers employing a directly installed classifier.
The production of coarse salts, requires a high slurry density of over 30%, in order to limit nuclei formation. Slurry density is determined by dividing the weight of salt by the weight of the salt and the weight of solution.
German Pat. No. 1,297,079 teaches obtention of coarse salts from solutions and more specifically the extraction of coarse KC1. In the process described in German Pat. No 1,297,079, a crystallizer with a directly connected classifier located above the crystallizer is utilized. Coarse KC1 suitable for use in fertilizer is produced from concentrates containing 5 to 40% by weight of solid content.
For crystallization of coarse K.sub.2 SO.sub.4, which is used as a fertilizer, preferably in chlorine-free form, a greater slurry density, over 40% by weight is required.
In the known crystallizers this high slurry density may be adjusted, but the continuous withdrawal of salt to a directly mounted classifier such as the classifier of German Pat. No. 1,297,079 detrimentally affects separation sharpness below in the crystallizer because of the formation of packs of (slurry) above the extension of the classifier pipe. The effective classifying section of the classifier is limited thereby to the uppermost part of the classifier pipe and the lower conical part of the crystallizer.
The prior art has the disadvantage that salts with a high content of fine grains below 0.2 mm are present in the classified salt after separation.
The foregoing disadvantages of the prior art are overcome by the process of this invention wherein coarse K.sub.2 SO.sub.4 is obtained utilizing a crystallizer with interior circulation. This is accomplished by removing the salt from the crystallizer for classification and utilizing for classification, a classifier which is separate from the crystallizer.
The process of the invention permits the obtention of coarse K.sub.2 SO.sub.4 with a high order of selectivity.